264 CHEMICAL ENGINEERING
cone returns to the mill. The Raymond or similar processes is adapted to the grinding of phosphate rock, pigments, amorphous graphite, talc, soapstone, fuller's earth, bone black, shellac, bark, roots, herbs and drugs, bone, etc.
Air separation can be done by either blowing on the material to be separated or by suction. Separation by aspiration is usually to be preferred to directing the blast against the material to be separated as this mode tends to disturb the larger particles and even to carry them up into the air current.
In the separation of asbestos from gangue by aspiration methods the fluffy particles of asbestos are the ones taken up by the air current, the sand and refuse falling back in the separating devices and either going to waste or being further treated to yield inferior grades of asbestos.
Separation with Air as an Adjunct.—Two devices for separating by the aid of air currents passed up through a bed or mass of material will illustrate the principles on which these machines operate. These machines should be described under the head of "Concentration7' as they employ principles of separation which have received their greatest development in that art. But since in the solution of some problems for which they are advocated there are but slight differences of specific gravity in the parts to be separated it is thought best to treat of them here.
Middlings Purifier.—For the purposes of mechanical separation the wheat berry may be considered to consist of a center of starchy material or white flour and a coat wrapping this and containing a larger proportion, of gluten, which substance being absent in the loaf there results heaviness and sogginess. The deficiency of gluten in barley renders it unfit for bread making. There is finally bran surrounding the gluten coat. After wheat receives its first rolling there results much of the weak flour from the central portion of the berry, some grains of the coat of gluten and some gluten grains with bran attached and finally, bran. The weak flour is readily removed by sifting. The oversize of the sifting or middlings contains all the parts of the berry which will make the strongest and best flour provided the bran can be removed. The problem of purifying the middlings was solved in the 60's of the last century by the middlings purifier invented by LaCroix, Smith and others, the appearance of the purifier being followed by much bitter litigation and strife over the priority of the invention. Smith's purifier which has existed with little change since his time consists of a flat shaking screen in an enclosed box.
The middlings are fed in at one end of the screen by means of a roller feeder so that there will be no escape of air at that point. The long relatively narrow screen is actuated by an eccentric outside the box and the middlings are progressed by the differential secured by inclining the rock legs which support the screen. During the progression along the screen the bed of middlings is subjected to an air current which passes up through them and the screen, this current being created by a fan placed in the top of the box. Mounted on the screen frame are sections of screen cloth the meshes of which sections become progressively coarse from the feed end to the lower end of the screen frame. The enclosing box is also divided internally by baffles into compartments so that there may be individual regulation of the air currents for each screen size. To regulate the air currents gates are placed in each compartment at either the top or bottom of the enclosing box. Originally the regulating gates were placed in the top of the device but the bottom is now the preferred position. If the gates be placed in the top of the box there is danger of material adhering to the gate passages and falling back on the screen. The single fan draws the air up through